@benlo626
First thing you need is to find where your sia-UI config/data folder is. That is the one containing following subdirectories:
consensus
gateway
wallet

I hope you know how to search your linux machine for a folder named "consensus" or "wallet".

To delete your Sia wallet you just delete the folder named "wallet" (kill the siad process before that).
To completely delete your "Sia account", delete all contents of the folder where you found these subfolders. And burn the paper where you wrote down your seed.
Though, the "account" (wallet) will remain on the blockchain, you (or anyone else) will never use it.

Yes, but...if you're mining in a pool, then the unpaid balance mined on the previous wallet address will not be converted to the new address. So finish mining on the old address until it's paid out, then switch to the new address.

Unless you have completely lost hope in polo, in which you need to accept the loss and move on immediately! :-)